202 ANIMALS AT WORK AND PLA Y 



do so, they find their way across seas and lands, 

 unknown to many of their number, to a goal 

 which, in the case of young birds of the year re- 

 treating to their winter homes, must be unknown, 

 and which, in the case of their first return to the 

 land in which they were bred, can at best be but 

 an imperfect memory. One hypothesis supposes a 

 reason for the northern migration of birds in the 

 desire of species to return to districts from which 

 they were formerly driven by the advance of ice in 

 the glacial age. Mr C. Dixon, in a recently 

 published book on the Migration of British Birds, 

 suggests another. He notes that all migration has 

 for its object to increase the breeding area of a 

 species, and that the tendency of all birds is to 

 move in the direction of the Poles, which tendency 

 he calls the 'Law of Dispersal.' Thus, in our 

 hemisphere, there is no known migration route 

 which goes south in summer or north in winter. 

 Hence, our migratory birds winter in the countries 

 in which their ancestors escaped extermination in 

 the glacial epoch, and go north to extend their 

 breeding area at a time when the individuals need 

 more space and food than at any other time, in 

 obedience to instinct used strictly in Paley's sense 

 of a ' propensity prior to experience and independent 

 'of instruction,' for Mr Dixon can hardly mean that 



